1. Field of the Invention
The invention relates to a control apparatus and a method of controlling an internal combustion engine mounted on a vehicle.
2. Description of the Related Art
During idle operation of an internal combustion engine mounted on a vehicle, such as an automobile, an engine Idle Speed Control (ISC) for adjusting the engine speed to a target engine speed set in accordance with the engine operation state, such as an engine temperature, is performed. The target engine speed of the engine idle speed control is set as follows based on, for example, the engine temperature. That is, the target engine speed of the engine idle speed control is set higher when the engine temperature is low, and hence the viscosity of lubricating oil is high, than when the engine temperature is high, and hence the viscosity of the lubricating oil is low. This is because, as the viscosity of the lubricating oil for the internal combustion engine increases, the rotational resistance of the engine increases and the likelihood that the engine will stall during idle operation, which results from the increased rotational resistance, needs to be suppressed. Further, when the internal combustion engine is idling, the engine speed reduction prevention control for rapidly increasing the engine speed to suppress the stalling of the engine when the engine speed is equal to or below a threshold engine speed, which is lower than the target engine speed, is executed as well (see Japanese patent application publication No. 2006-46263 (JP-A-2006-46263) (paragraphs [0030] and [0036])).
If the engine temperature is low and the target engine speed of the engine idle speed control is set high when the speed of the vehicle is equal to or lower than a predetermined vehicle speed to shift the engine operation to the idle operation in the process of stopping the vehicle from running through the action of a brake, the engine speed during idle operation is also set high accordingly. In this case, the driving force applied to driving wheels of the vehicle at that moment becomes large as a result of the engine speed during idle operation that is set high as described above. Even when a braking force is applied to the driving wheels through the brake, the rotational speed of the driving wheels is unlikely to be reduced. Thus, it takes some time to stop the vehicle from running.
To cope with such a situation, in the process of stopping the vehicle from running, the transmission may be held in a neutral state to prevent the driving force from being applied to the driving wheels of the vehicle, as described in Japanese patent application publication No. 8-74992 (paragraphs [0032] to [0034]). In this case, even if the engine speed is adjusted to the target engine speed, which has been set high, upon a shift of engine operation to idle operation that results from the vehicle speed that has become equal to or lower than the predetermined value, the driving force based on engine speed at that moment is not transmitted to the driving wheels. It is therefore assumed that the vehicle can be swiftly stopped from running after the shift to idle operation. However, in the process of stopping the vehicle from running before the shift to idle operation, the transmission is held in the neutral state and hence a driving force transmission path between each of the driving wheels and the internal combustion engine is shut off. Thus, the rotational resistance of the internal combustion engine does not act as a braking force for the driving wheels, and the braking force for stopping the vehicle from running is provided entirely by the brake alone. However, it is possible that the brake alone will not be able to provide the required braking force. If the braking force cannot be ensured with ease, it takes longer time to stop the vehicle from running.
Further, instead of holding the transmission in neutral in the process of stopping the vehicle from running, it is also conceivable to forcibly reduce the target engine speed of the engine idle speed control upon a shift of engine operation to idle operation, and thereby reduce the engine speed during the engine idle operation so that the driving force applied to the driving wheels on the basis of engine speed is held small. In this case, in the process of stopping the vehicle from running before the shift to idle operation, the rotational resistance of the internal combustion engine acts as a braking force for the driving wheels. Thus, the braking force for swiftly stopping the vehicle from running may be applied to the driving wheels through the rotational resistance of the internal combustion engine and the brake. Further, after the shift of engine operation to idle operation in the process of stopping the vehicle from running, the driving force applied to the driving wheels may be held small by forcibly reducing the target engine speed used in the engine idle speed control. Thus, the vehicle is swiftly stopped from running.
As described above, the vehicle is swiftly stopped from running by forcibly reducing the target engine speed of the engine idle speed control upon a shift of engine operation to idle operation in the process of stopping the vehicle from running.
However, when the target engine speed is forcibly reduced, it becomes close to the threshold engine speed of the engine speed reduction prevention control. Thus, in adjusting the engine speed to the reduced target engine speed through engine idle speed control, there may be a disturbance acting on the driving wheels in such a direction as to stop rotation thereof. For example, an external force (a frictional force or the like) applied from a road surface side to the driving wheels may act reversely to the rotational direction of the driving wheels. When the disturbance acts on the driving wheels, the engine speed may be reduced below the threshold engine speed. Then, when the engine speed becomes lower than the threshold engine speed, a rapid rise in the engine speed is caused through engine speed reduction prevention control for suppressing the stalling of the internal combustion engine. Thus, the driving force applied to the driving wheels based on the engine speed becomes larger than the braking force. As a result, it becomes difficult to swiftly stop the vehicle from running.